Science educators incorporate collaborative engagement in model-based argumentation to meet curricular goals and build students' capacity for scientific epistemic and social practices. During collaboration, groups encounter various challenges (e.g., lack of task understanding) and engage in social regulation to overcome them. However, little is known about whether and how various contextual factors (e.g., teacher presence, classroom climate, or academic discipline) can influence the nature of students' collaboration and social regulation. The purpose of our qualitative case study was to examine how one such contextual factor, teacher presence, related to high school students' discourse interactions and social regulation of learning during a collaborative model-based scientific argumentation task. In one classroom, the teacher was continuously present during groups' discussions. In the other classroom, the teacher was intermittently present. We found that groups with a continuously present teacher had high on-task engagement that was teacher-led, with the students relying on the teacher for regulation. Groups with intermittent teacher presence had more off-task interactions but also engaged in more dialogic argumentation discourse with each other, initiating and enacting more modes of social regulation of learning than the other groups. These findings suggest that teachers must thoughtfully manage their presence and absence to instruct, model, scaffold, and fade their support for both scientific argumentation and the social regulation skills necessary to productively enact that argumentation, intentionally varying emphasis on one or the other. These findings highlight the importance of future research on teacher presence and other contextual factors that can affect how students collaborate and learn.